The receive band portion of the North American cellular spectrum extends from 824 MHZ to 849 MHZ and is divided up into "A" and "B" bands for wireline and non-wireline operators. This includes a recent extension of the spectrum by an additional 5 MHZ of bandwidth. Due to legacy considerations, the spectrum extension was done in noncontiguous bands. As a result, the "A" band now consists of an A"A band extending from 824 MHZ to 835 MHZ together with an A' band extending from 845 MHZ to 846.5 MHZ.
Receivers are thus required to perform an analog-to-digital conversion on frequencies within a 22.5 MHZ band extending from 824 MHZ to 846.5 MHZ. State of the art ADCs (analog-to-digital converters) of sufficient dynamic range are capable of sampling at a sampling frequency (FS) of 65 mega samples per second. Using the lowest conversion band, (first Nyquist, frequency&lt;FS/2), a 32.5 MHZ band can be converted.
One existing solution to dealing with the new noncontiguous A band involves the direct conversion of the entire 22.5 MHZ band. However, converting the entire band such that the highest frequency component is under 30 MHZ means that the lowest 30 KHz channel will be at or below 7.5 MHZ. Harmonics of these lowest channels generated in the analog circuits before the ADC would fall at 15, 22.5 and 30 MHZ. Hence these harmonics will interfere with other A band channels and/or B channels in the case that the receiver is tuned to receive B band signals, and this is undesirable.
A second approach is to fold the A' band such that at baseband, the A' band will result in a frequency close to the converted A"A band. With this approach, IF filter rejection is insufficient to prevent B band frequencies from falling on top of A'. This is particularly true in the filter guard band. In addition, a performance degradation is suffered by combining similar frequencies in a high linearity analog line-up.
U.S. Pat. No. 5,668,836 by Smith et al. which issued Sep. 16, 1997 entitled "Split Frequency Band Signal Digitizer and Method" discloses a receiver in which the entire A band is downconverted in a single step such that the A"A band is below half the digitizer sampling frequency FS, and the A' band is above half the digitizer sampling frequency. This signal is digitized to produce a digitized signal having a spectrum containing a replica of the A' signal in the band of interest, namely that below half the digitizer sampling frequency, due to aliasing. This approach has several problems. Firstly, it appears that the A' band ends up being folded over on top of the B band, and no mechanism for the adequate removal of the B band is suggested. Secondly, it appears likely that at least a portion of the A band will end up below FS/4 and thus will produce in-band harmonics (harmonics below FS/2). Thirdly, the frequency axis of the A' band is flipped, in other words the I and Q sidebands are reversed. This requires a modification to the hardware or software components performing demodulation.
It would be of great commercial advantage to be able to convert all of the A band at the same time and all of the B band at the same time, but not necessarily to convert all of A and B at the same time, and without causing a flip in the frequency axis, or causing the B band to interfere with the converted A band.